This invention relates to glass-to-metal seals and, more particularly, to glass-to-metal seals using glass compositions with high thermal expansion coefficients and good resistance to Li corrosion, and high expansion metal pin materials. These seals can be used for the glass-to-metal seals in components exposed to severe chemical environments, e.g. in headers for ambient temperature Li batteries.
Lithium ambient temperature batteries provide high energy densities and high rate capabilities, at low temperatures; however, a major problem associated with these cells is the highly corrosive nature of Li chemistry. Standard glass insulators, used to separate the body of a battery header from the center pin while providing a hermetic seal for the battery, experience extensive corrosion over relatively short periods of time, thus severely limiting the shelf life of the cells.
An additional problem associated with conventional Li batteries is the current use of molybdenum as the pin material for center pins in Li battery headers. Molybdenum is a difficult material to work with, being difficult to weld, difficult to machine as it is very brittle, and susceptible to aqueous corrosion. It is desirable to use alternative pin materials, instead of molybdenum. Replacement of molybdenum with more weldable, more machinable, and more chemically resistant alloys would improve both the ability to manufacture Li batteries and their ultimate performance. Improved pin materials for lithium batteries include Alloy-52 and 446 stainless steel. Alloy 52 is a Ni--Fe alloy also referred to as Niron 52; Niron 52 is described in Hawley, G. G., The Condensed Chemical Dictionary, Tenth Edition, Van Nostrand Reinhold Company, New York, New York, 1981, page 728, as the trademark for a magnetic alloy of 50% nickel and 50% iron, having the following properties: density of 8.46 gm/cm.sup.3, and tensile strength of 70,000 psi. Alloy 52 or Niron 52 has a linear thermal expansion coefficient of 98.times.10.sup.-7 /.degree.C. at room temperature to 500.degree. C. The trademark 446 stainless steel is the AISI designation for a ferritic steel which is magnetic and non-heat treatable, having a linear thermal expansion coefficient of 114.times.10.sup.-7 /.degree.C. at room temperature to 700.degree. C.; also, 446 stainless steel is defined in Clauser, Henry R., Encyclopedia/Handbook of Materials, Parts, and Finishes, Technomic Publishing Company, Inc., Westport, Connecticut, 1976, pages 446-447, as a ferritic steel with 23-27% chromium content, 0.20 maximum % carbon content, and 0.25 maximum % nitrogen content, typically used in applications requiring high resistance to corrosion and oxidation.
In order to form an acceptable glass-to-metal seal in an ambient temperature Li battery, the glass must meet three main criteria. First, it must have a high resistance to Li corrosion; second, it must be able to make a hermetic seal between the metal header body and the metal center pin, which requires an expansion match between the glass and the pin; and, third, it must be an electrical insulator so that the header body and the center pin are electrically isolated.
The glass currently used in the glass-to-metal seal in headers for Li ambient temperature batteries is TA-23, which has a finite corrosion rate when in contact with Li metal. Although TA-23 glass resists Li corrosion well enough to project battery lifetimes out to 5-10 years, the finite corrosion rate limits the lifetime of the battery.
Glasses based on the CaO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 and CaO--MgO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 systems have been developed to improve the corrosion resistance and extend the battery lifetime. The most promising of these latter glasses is CABAL-12 which, like TA-23, is designed to have a thermal expansion coefficient that closely matches that of the molybdenum center pin, about 60.times.10.sup.-7 /.degree.C. CABAL-12 has far superior corrosion resistance than TA-23, but all of the CaO-Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 and CaO--MgO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 glasses have limited thermal expansion coefficient ranges, on the order of 60-90.times.10.sup.-7 /.degree.C., which makes them unsuitable for sealing to the high expansion, alternative metal, pin materials.
There is no previously established technology that describes hermetic, electrically insulating, glass-to-metal seals for Li batteries, using non-silicate sealing glasses and center pins of materials other than molybdenum.
There is an existing need for the development of such technology to allow the replacement of the molybdenum center pin, currently used in most commercial Li batteries, with alternative pin materials that are easier to weld, are easier to machine, and are less susceptible to aqueous corrosion.
Also, there is an existing need for the development of hermetic glass-to-metal seals for lithium-power supplies, using nonsilicate glasses, which resist attack by lithium at least as well as currently used glasses, e.g. TA-23 and CABAL-12, with center pins of other materials than molybdenum.
There is a still further need for the development of hermetic glass-to-metal seals for Li-chemistry power supplies using aluminoborate glasses having thermal expansion coefficient ranges of 90-120.times.10.sup.-7 /.degree.C., that are as resistant to attack by lithium and other aqueous and electrolytic solutions as are currently used glasses such as TA-23, and that can be sealed with pin materials other than molybdenum, such as Alloy 52 and 446 stainless steel, using conventional sealing technology.